This application is based on Japanese patent application No. 10-283917 filed on Oct. 6, 1998, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a surface light source device and a film scanner using the same.
2. Description of the Related Art
Conventionally, surface light source devices have been used as backlights of flat-panel displays typified by liquid crystal displays and signboards and as devices for backlighting film images in film scanners for reading film images.
Japanese Unexamined Laid-open Patent Application No. 09-33923 discloses a surface light source device for backlighting. Referring to FIG. 8, this surface light source device comprises a light directing plate 100, a radiation light source 101 and a reflector 107. The radiation light source 101 is tubular, and disposed at a side of the light directing plate 100. Light from the radiation light source 101 is incident on the light directing plate 100 directly and after reflected at the reflector 107. The incident light and the light reflected at a reflecting surface 102 provided on an incidence opposite surface of the light directing plate 100 are diffused by a diffusing pattern 104a provided on an exit opposite reflecting surface 104 and a diffusing plate 106 provided on an exit surface 105. Consequently, the light from the radiation light source 101 is directed to the exit surface 105 without any nonuniformness. color surface light source device for backlighting. Referring to FIG. 9, in this color surface light source device, light from LEDs 200 is made incident from a side of a box 201, and after reflected at a reflecting plate 202 in the box 201, the light is made to impinge on a diffusing plate 203 so as to be diffused. Consequently, the light from the LEDs 200 exits from an exit surface 204 without any nonuniformness.
In the conventional example shown in FIG. 8, however, since the reflector 107 and the light directing plate 100 are used to effectively use the light from the radiation light source 101, the structure is complicated and the cost is high. Moreover, when a fluorescent tube is used as the radiation light source 101, it is necessary to increase the operating voltage to approximately 24 V. Further, when a fluorescent tube is used, an inverter circuit is necessary to suppress illumination flicker. For illuminating small areas such as film images, this device is unsuitable because the cost is too high. In addition, the running cost is high. When a xenon tube is used instead of a fluorescent tube, since it is necessary to accumulate charges in a capacitor by increasing the voltage like in the case of a fluorescent tube, the cost is somewhat high although it is not as high as that in the case of a fluorescent tube. Moreover, since it is impossible for this light source device to be always on, it is necessary to control the timing of taking in charges. Since fluorescent tubes and xenon tubes are still thick, they are unsuitable for size reduction, particularly, thickness reduction of surface light source devices.
In the conventional example shown in FIG. 9, since the light from the LEDs 200 is bent only once and is diffused only once, it is necessary for the optical path to be long in order to obtain uniform plane light. Thus, this light source device is also unsuitable for size reduction.
On the other hand, a white LED was recently invented and is attracting attention as a lighting source. This lighting source whose working voltage is approximately 4 V is excellent in energy conversion efficiency and never breaks down.
Therefore, the inventors prototyped a surface light source device using this lighting source. However, the light emission quantity is limited, and in the system shown in FIG. 9 in which the LEDs having high diffusibility is used and the light is made incident from a side of the light directing plate, a sufficient light quantity cannot be obtained. Consequently, to obtain a necessary quantity of plane light, it is necessary to provide LEDs all over a necessary area, so that the cost increases.
An object of the present invention is to provide a small-size, low-cost surface light source device with which a sufficient quantity of uniform plane light is obtained, and a film scanner using the same.
Another object of the present invention is to provide a surface light source device with which a sufficient quantity of plane light is obtained even when a low-illumination light source is used, and a film scanner using the same.
To achieve at least one of the above-mentioned objects, a surface light source device of the present invention essentially comprises a plate optically disposed between a light source and a light exit surface, said plate being capable of partly passing a light therethrough and of partly diffusing the light. The plate is preferably provided so as to be inclined against at least one of the light source, the light exiting surface and a reflecting surface for reflecting the light.
For example, the surface light source device of the present invention is specifically provided with: a light source; a first reflecting surface which is provided so as to be vertical to an optical path from the light source; a second reflecting surface which is provided between the light source and the first reflecting surface so as to be parallel to the optical path; an exit diffusing plate; and a transparent thin plate tilted or inclined with respect to the optical path in a direction such that light from the light source is reflected toward the second reflecting surface, and where a diffusing pattern is provided.
According to this specific structure, the light from the light source is partly transmitted by the transparent thin plate disposed on the optical path thereof at transparent parts where there is no diffusing pattern, and reaches the first reflecting surface serving as an exit opposite surface to be reflected toward an incidence surface. However, the ratio of the light from the light source directly reaching the first reflecting surface is halved because of the light interrupted by the diffusing pattern of the transparent thin plate and the light reflected by the transparent thin plate toward the second reflecting surface. Since the light reaching the first reflecting surface to be reflected toward the incidence surface again reaches the transparent thin plate, the ratio of the light directly passing through toward the incidence surface is further halved because of the light similarly interrupted by the diffusing pattern and the light surface-reflected at the transparent thin plate toward the exit surface. Consequently, the effectiveness of light improves even when there is no reflecting surface on the side of the incidence surface, and a sufficient quantity of exit light is obtained even when the LED is used as the light source, so that characteristics of the LED are delivered that the life is long and that the device cost and the running cost are both reduced. Further, the light from the light source is diffused by the diffusing pattern of the transparent thin plate three or more times including when the light reaches the transparent thin plate for the first time, when the light is transmitted by the transparent thin plate toward the first reflecting surface to be reflected at the first reflecting surface and again reaches the transparent thin plate, and when the light from the light source reaches the transparent thin plate for the first time to be reflected toward the second reflecting surface and the reflected light is then reflected at the second reflecting surface to again reach the transparent thin plate, and the light is further diffused by the exit diffusing plate when exiting from the exit surface, so that the diffusibility of the exiting light is high. Consequently, uniform plane light is obtained although of the LEDs, one that easily effectively uses light and has an excellent directivity is used as the light source. Therefore, the light diffused by the transparent thin plate is directed in all directions, and although some of the light exits outside from the incidence surface, this hardly affects the quantity of exit light because the ratio of the light exiting outside is low and the remaining light reaches the exit surface. Moreover, since the optical axis of the light source is parallel to the exit surface, the size in the direction of the thickness as the surface light source device like in the case in which the light source is tilted is small, and since the transparent thin plate having a diffusing pattern is only obliquely disposed instead of a light directing plate, the size is small and the cost is low. Therefore, this structure is suitable for illuminating small areas such as film images. In addition, weight reduction is achieved.
The number of light sources may be one. However, according to the size of the area to be illuminated, a plurality of light sources such as LEDs may be disposed parallel to the exit surface.
When the light source, the first reflecting surface, the second reflecting surface, the exit diffusing plate and the transparent thin plate are held in one holder, the structure is simple, the positioning among the elements is easy and the surface light source device is convenient for use as a unit to be replaced.
Moreover, according to the present invention, to construct a film scanner for sensing film images illuminated from behind by a surface light source, the surface light source for performing the above-mentioned illumination is disposed in the rear of the film. Film images can highly precisely be read out with a sufficient quantity of uniform plane light capitalizing on the characteristics of the above-described surface light source device, and further, the overall size is small, the cost is low and the weight is light, and one in which LEDs are used as the light source and a plurality of LEDs are arranged in a direction parallel to the exit diffusing plate is suitable for the above-mentioned reasons.
Other objects and features of the present invention will become apparent from the following detailed description and drawings. The features of the present invention can be used independently or in various combinations to the greatest extent practicable.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrates a specific embodiment of the invention.
FIGS. 1(a) and 1(b) are cross-sectional views showing the structures of prototypes of a surface light source device of the present invention;
FIG. 1(c) is a cross-sectional view showing a best mode of the surface light source device of the present invention;
FIG. 1(d) is diagram showing the transparent thin plate 10 of FIG. 1(c).
FIG. 2 is a front view of the surface light source device of FIG. 1(c);
FIG. 3 is a longitudinal cross-sectional view showing an embodiment of a film scanner using the surface light source device of FIG. 1(c);
FIG. 4 is a lateral cross-sectional view of the film scanner of FIG. 3;
FIG. 5 is a rear view of the film scanner of FIG. 3 from which the exterior case of the body is removed;
FIG. 6 is a longitudinal cross-sectional view of the film scanner of FIG. 3 viewed from a different position from that of FIG. 3;
FIG. 7 is a perspective view showing another embodiment of the structure of connection between a magnetic recording head and a signal plate;
FIG. 8 is a cross-sectional view showing an example of conventional surface light source devices; and
FIG. 9 is a cross-sectional view showing another example of conventional surface light source devices.